Which type of authentication is commonly used with SMTP servers?

Basic authentication and OAuth are commonly used methods for authenticating with SMTP servers.

Basic authentication is a straightforward method where the client sends the username and password encoded in Base64. Although it's easy to implement, it is not the most secure method unless used in conjunction with SSL/TLS to encrypt the transmission, ensuring that sensitive information is not transmitted in plain text.

OAuth, on the other hand, is a more secure and flexible protocol that allows for token-based access without exposing usernames and passwords. With OAuth, users can grant third-party applications limited access to their accounts without sharing their credentials, which enhances security and user privacy.

Together, these methods provide SMTP servers with a foundation for verifying the identity of email clients attempting to send messages, ensuring that only authorized users can access the server and preventing unauthorized email sending, which is essential for maintaining the integrity and security of email communications.